/*
 * Copyright (C) 2016 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.graph;

import java.util.Map;
import javax.annotation.Nullable;

/**
 * A {@link MapIteratorCache} that adds additional caching. In addition to the caching provided by
 * {@link MapIteratorCache}, this structure caches values for the two most recently retrieved keys.
 *
 * @author James Sexton
 */
class MapRetrievalCache<K, V> extends MapIteratorCache<K, V> {
    @Nullable
    private transient CacheEntry<K, V> cacheEntry1;
    @Nullable
    private transient CacheEntry<K, V> cacheEntry2;

    MapRetrievalCache(Map<K, V> backingMap) {
        super(backingMap);
    }

    @SuppressWarnings("unchecked") // Safe because we only cast if key is found in map.
    @Override
    public V get(@Nullable Object key) {
        V value = getIfCached(key);
        if (value != null) {
            return value;
        }

        value = getWithoutCaching(key);
        if (value != null) {
            addToCache((K) key, value);
        }
        return value;
    }

    // Internal methods ('protected' is still package-visible, but treat as only subclass-visible)

    @Override
    protected V getIfCached(@Nullable Object key) {
        V value = super.getIfCached(key);
        if (value != null) {
            return value;
        }

        // Store a local reference to the cache entry. If the backing map is immutable, this,
        // in combination with immutable cache entries, will ensure a thread-safe cache.
        CacheEntry<K, V> entry;

        // Check cache. We use == on purpose because it's cheaper and a cache miss is ok.
        entry = cacheEntry1;
        if (entry != null && entry.key == key) {
            return entry.value;
        }
        entry = cacheEntry2;
        if (entry != null && entry.key == key) {
            // Promote second cache entry to first so the access pattern
            // [K1, K2, K1, K3, K1, K4...] still hits the cache half the time.
            addToCache(entry);
            return entry.value;
        }
        return null;
    }

    @Override
    protected void clearCache() {
        super.clearCache();
        cacheEntry1 = null;
        cacheEntry2 = null;
    }

    private void addToCache(K key, V value) {
        addToCache(new CacheEntry<K, V>(key, value));
    }

    private void addToCache(CacheEntry<K, V> entry) {
        // Slide new entry into first cache position. Drop previous entry in second cache position.
        cacheEntry2 = cacheEntry1;
        cacheEntry1 = entry;
    }

    private static final class CacheEntry<K, V> {
        final K key;
        final V value;

        CacheEntry(K key, V value) {
            this.key = key;
            this.value = value;
        }
    }
}
